System for indicating the position of a surgical probe within a head on an image of the head

ABSTRACT

A system for determining a position of a probe relative to an object such as a head of a body of a patient. The head includes a surface such as a forehead having a contour. The head is placed in a cradle equipped with an arc. During surgery, an optical scanner determines the position of the forehead relative to a base ring. An array for receiving radiation emitted from the probe and from the base ring generates signals indicating the position of the tip of the probe relative to the base ring.

This application is a continuation of prior U.S. patent application Ser.No. 08/543,516 which was filed on Oct. 16, 1995 (issued as U.S. Pat. No.5,851,183). Which is a continuation of Ser. No. 07/858,980 filed May 15,1992 which is a Ser. No. 07/600,753 filed Oct. 19, 1990 now abandoned.

BACKGROUND OF THE INVENTION

Precise localization of position has always been critical toneurosurgery. Knowledge of the anatomy of the brain and specificfunctions relegated to local areas of the brain are critical in planningany neurosurgical procedure. Recent diagnostic advances such ascomputerized tomographic (CT) scans, magnetic resonance imaging (MRI)scanning, and positron emission tomographic (PET) scanning have greatlyfacilitated preoperative diagnosis and surgical planning. However, theprecision and accuracy of the scanning technologies have not becomefully available to the neurosurgeon in the operating room. Relatingspecific structures and locations within the brain during surgery topreoperative scanning technologies has previously been cumbersome, ifnot impossible.

Stereotactic surgery, first developed 100 years ago, consists of the useof a guiding device which channels the surgery through specific parts ofthe brain as localized by preoperative radiographic techniques.Stereotactic surgery was not widely used prior to the advent of modernscanning technologies as the injection of air into the brain wasrequired to localize the ventricles, fluid containing chambers withinthe brain. Ventriculography carried a significant complication rate andaccuracy in localization was marginal.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a system which candetermine the position of a probe within a head and display an imagecorresponding to the determined position.

The invention comprises a system for determining a position of a tip ofa probe, which is positioned within an object, relative to crosssectional images of the object. The system comprises measuring means,translating means and selecting and displaying means. The measuringmeans measures the position of the tip of the probe relative to theobject. The translating means translates the position of the tip of theprobe relative to the object into a coordinate system corresponding tothe cross sectional images of the object. The selecting and displayingmeans selects the image of the object which corresponds to the measuredposition of the tip of the probe relative to the object and displays theselected image.

The invention also comprises a system for determining a position of atip of a surgical probe, which is positioned within a head of a body ofa patient, relative to cross sectional images of the head. Meansmeasures the position of the tip of the surgical probe relative to thehead. Means translates the position of the tip of the surgical proberelative to the head into a coordinate system corresponding to the crosssectional images of the head. Means selects the image of the head whichcorresponds to the measured position of the tip of the surgical proberealtive to the head and displays the selected image.

The invention also comprises a method for determining a position of atip of a surgical probe, which is positioned within a head of a body ofa patient, relative to cross sectional images of the head, said methodcomprising the steps of: measuring the position of the tip of thesurgical probe relative to the head; translating the position of the tipof the surgical probe relative to the head into a coordinate systemcorresponding to the cross sectional images of the head; selecting theimage of the head which corresponds to the measured position of the tipof the surgical probe relative to the head; and displaying the selectedimage.

The invention also comprises a system for determining a position of anultrasound probe relative to a head of a body of a patient when theprobe is positioned adjacent to the head. An array is positionedadjacent the probe. First means determines the position of theultrasound probe relative to the array. Second means determines theposition of the head relative to the array. Means translates theposition of the ultrasound probe into a coordinate system correspondingto the position of the head.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective illustration of a cylindrical frame structurewhich is mounted around a patient's head during the scanning process.

FIG. 1B is a plan view of the rods of the cylindrical frame structure ofFIG. 1A taken along a plane midway between the upper and lower rings.

FIG. 1C is a perspective illustration of a reference ring which ismounted by uprights to a patient's head to support the cylindrical framestructure of FIG. 1A.

FIG. 1D is a perspective illustration of the coordinate system of athree dimensional scanned image.

FIG. 2A is a perspective view of the caliper frame used to determine therelative position between a position in the head and the phantom base.

FIG. 2B is a perspective view of the caliper frame of FIG. 2Aillustrating its angles of adjustment.

FIG. 2C is a block diagram of the steps involved in the prior artprocess of determining the position of surgical probe relative to thescanned images so that the image corresponding to the probe position canbe identified and viewed by the surgeon.

FIG. 2D is a perspective illustration of a three dimensional coordinatesystem of a surgical probe.

FIG. 3A is a block diagram of a system according to the invention forindicating the position of a surgical probe within a head on an image ofthe head.

FIG. 3B is a perspective schematic diagram of the microphone array,surgical probe and base ring according to the invention.

FIG. 3C is a block diagram of the steps involved in the processaccording to the invention for determining the position of a surgicalprobe relative to the scanned images so that the image corresponding tothe probe position can be identified and viewed by the surgeon.

FIG. 3D is a perspective schematic diagram of an optical scanner used incombination with a cradle.

FIG. 3E is a perspective schematic diagram of the microphone array,surgical probe, base ring and optical scanner according to theinvention.

FIG. 4 is a flow chart of the translational software for translatingcoordinates from the surgical probe coordinate system to the scannedimage coordinate system according to the invention.

FIG. 5A is a perspective schematic diagram of an ultrasound probe systemaccording to the invention;

FIGS. 5B and 5C illustrate scanned ultrasound images, respectively.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With the advent of modern scanning equipment and techniques, severalstereotactic systems have been developed and are presently available.These stereotactic systems allow a surgeon to localize specific pointsdetected on CT, MRI or PET scans which have been previously generatedprior to the surgical procedure being performed. In particular, thestereotactic systems allow the selection of specific points detected onthe scans to be localized within the brain by the surgeon during thesurgical procedure using a mechanical device.

Initially, prior to the operative procedure, some form of localizingdevice, such as a frame, is attached to the patient's skull using sharppins. The particular scan or scans which are to be performed are thengenerated with the head of the patient encircled by the frame. Forexample, the frame may be comprised of a cylindrical structure 100 asillustrated in perspective in FIG. 1A. Structure 100 includes an uppercircular ring 102 and a lower circular ring 104 which are interconnectedby six vertical rods 106 and three diagonal rods 108. The three diagonalrods 108 diagonally interconnect rings 102 and 104 so that any planewhich passes through the cylindrical structure 100 and orthogonallyintersects its axis 108 will intersect each of the diagonal rods 108 ata particular point. The resultant spacing between the diagonal andupright rods defines a unique plane within the cylindrical structure100. For example, as shown in FIG. 1B, a scan in a particular planewould show a pattern of nine cross sectional views of the rods 106. Theunique spacing of these views of the rods, as shown in plane 112 of FIG.1B, would necessarily indicate that the position of the scan plane 112was parallel to and midway between rings 102 and 104 of the cylindricalstructure 100.

As a result of the scanning process, the images obtained are analyzedand the position within the images of the specific marking rods 106,called fudicels, are identified and measured. By measuring the distancebetween the rods 106, the specific location of a scan with reference toa base plane can be identified. Generally, the lower ring 104 of thecylindrical structure 100 is attached to a reference ring 120 (alsoknown as a BRW head ring) as illustrated in FIG. 1C. As noted above,this ring 120 is supported on the patient's head via uprights 122attached to the head by the use of sharp pins 124 so that the ring 120is held firmly in place with respect to the head. The lower ring 104 ofthe cylindrical structure 100 is mounted to the reference ring 120attached to the patient's head so that these two rings are in parallelplanes.

As shown in FIG. 1D, the scanning system (e.g., CT, MRI, PET) which isperforming the scanning has a scanned image coordinate system (X₀, Y₀,Z₀) within which a reference plane RP can be defined by at least threereference points RP1, RP2 and RP3 located on the head 124 of thepatient. A computer is then used to calculate a specific position withinthe brain and a target picked out on the specific image can beapproached with a fair degree of accuracy during the surgical procedure.

Although stereotactic surgery allows a surgeon to be guided to aspecific point with accuracy, it has not been particularly useful inallowing the surgeon to identify the particular location of a surgicalprobe within the brain at any point during the surgical process.Frequently in neurosurgery, brain tumors or other target points withinthe brain are indistinguishable from surrounding normal tissue and maynot be detected even with the use of frozen sections. Moreover, withmodern microsurgical techniques, it is essential that the neurosurgeonidentify specific structures within the brain which are of criticalfunctional importance to the patient. In addition, the boundaries ofthese structures must be accurately defined and specifically known tothe surgeon during the surgical process. In this way, these tissues willnot be disturbed or otherwise damaged during the surgical processresulting in injury to the patient.

In the past, the surgeon has been able to use the stereotactic system inreverse in order to permit the determination of the position of asurgical probe relative to the scanned images so the image correspondingto the probe position can be identified and viewed. However, going inreverse from the patient's brain backwards to find the position of thesurgical probe relative to the scan is a cumbersome and time-consumingprocess. Usually, a specially designed caliper frame 200, as illustratedin FIG. 2A, has to be attached to the ring 120 affixed to the patient'shead to determine the position of the surgical probe in the head. Forexample, suppose the surgeon desires to know the position of a tip 201of a probe 202 in the patient's head. First, the caliper frame 200 isfitted to the reference ring 120 affixed to the patient's head. Next,the position of probe 202 is positioned on arch 206 and the frame 200 isset to indicate the alpha, beta, gamma and delta angles on scales 208,210, 212 and 214 that the probe 202 defines with respect to the frame200, as shown in FIG. 2B. Next, the distance 216 from the tip of theprobe 202 to the arch 206 is determined.

The caliper frame 200 is then transferred and mounted to a phantom base250 in a manner as illustrated in FIG. 2A. The phantom base 216 has acoordinate system (X₁, Y₁, Z₁). Generally, the caliper frame 200identifies a point 201 over the phantom base 250. A pointing device 252is positioned to have its tip 254 at point 201. The X₁-Y₁ plane of thephantom base 200 corresponds to a plane parallel to the plane in whichthe reference points RP1, RP2 and RP3 are located. The (X₁, Y₁, Z₁)coordinates define the position of point 201. As a result, the positionof point 254 with respect to the X₁-Y₁ plane and, therefore, withrespect to the reference plane RP is now known. A computer can now beused to calculate the specific position within the brain and theparticular scan which corresponds to the calculated position can now beaccessed and viewed on a scanning system.

In summary, this prior art process as shown in FIG. 2C identifies thelocation of the tip 201 of the surgical probe 202 for the surgeon.Initally, the surgeon positions the probe 202 on the caliper frame 200,which is attached to the head, at the position desired within the head.The caliper frame 200 is then removed from the patient's head andtransferred to the phantom base 250. The pointing device 252 is thenpositioned at point 254 which is essentially coaxial with point 201 ofthe tip of the probe. The pointing device 252 then indicates theposition of the tip of the probe in the phantom base coordinate system(X₁, Y₁, Z₁). Finally, these coordinates are used to determine thescanned image coordinates (X₀, Y₀, Z₀) so that the image correspondingto the probe position can be displayed.

After this cumbersome and time-consuming process, the surgeon has nowdetermined the position of the tip 201 of the probe 202 with respect tothe scanned images and can now view the image corresponding to the probeposition to decide the next step in the surgical procedure. This entireprocess takes approximately ten to fifteen minutes and increases therisks of intraoperative contamination as the base of the calipers arenonsterile. Because of these considerations, stereotactic surgery is notcommmonly employed in most procedures. Furthermore, the minimal accuracyit affords is generally insufficient for modern microsurgicaltechniques. Consequently, stereotactic surgery is not generallyavailable to the majority of certain patients undergoing surgery.

Comparing FIGS. 1D and 2A, it can be seen that it is necessary for thesurgeon to know the specific location of the tip 201 of the surgicalprobe 202 with respect to the scanned image coordinate system (X₀, Y₀,Z₀) of the particular scans that were preoperatively performed. In otherwords, the surgical probe 202 has a particular coordinate system (Y2,Y2, Z2) which is illustrated in FIG. 2D. Ideally, the surgical probecoordinate system (X₂, Y₂, Z₂) must be related to the scanned imagecoordinate system (X₀, Y₀, Z₀) . The prior art as illustrated in FIG. 2Bhas suggested relating these coordinate systems via the phantom basecoordinate system (X₁, Y₁, Z₁). However, as noted above, thisrelationship process is inaccurate, time-consuming and cumbersome. Theinvention uses a 3D digitizer system to locate the position of the tip201 of the surgical probe 202 and to directly relate the surgical probecoordinate system (X₂, Y₂, Z₂) to the scanned image coordinate system(X₀, Y₀, Z₀).

In particular, an off-the-shelf, three dimensional sonic digitizer suchas Model GP-8-3D produced by Scientific Accessories Corporation is usedto determine the position of the probe. As shown in FIG. 3A, the 3Ddigitizer system includes a microphone array 300 which is generallymounted in the operating room on the ceiling or in some other positionso that it is in a line of sight with the surgical probe 302 that isbeing used. As will be described in greater detail below, the probe 302includes transmitters such as sound emitters thereon which interact withthe microphone array 300 so that the position of the tip of surgicalprobe 302 is known at any particular instant in time. The 3D digitizersystem also includes a temperature compensation emitter 304 associatedwith the microphone array 300. Furthermore, mounted to the ring 120(FIG. 1C) affixed to the patient's head is a base ring 306 which iscoaxial and parallel with the plane defined by reference ring 120. Thisbase ring 306 includes a plurality of transmitters as will be describedbelow which interact with the microphone array 300 so that the relativeposition of the base ring 306 can be determined any particular instantin time. Signal generator 308 generates a signal which is providedthrough a multiplexer 310 to the temperature compensation emitter 304,surgical probe 302, and base ring 306. Usually, temperature compensationemitter 304 is activated by the signal generator 308 via multiplexer 310to emit a signal which is received by the microphone array 300. Each ofthe signals received by each of the microphones of the array 300 isprovided to a digitizer 312 which digitizes the signals and provides thedigitized signals to computer 314 which includes a spatial acquisitionand recording (SAR) program 316 which acquires and records spatialcoordinates based on the digitized signals. For example, program 316 maybe the SACDAC program licensed by PIXSYS of Boulder, Colo. This programevaluates the digitized signals emitted by the temperature compensationemitter 304 to determine the reference standards. i.e., the velocity ofthe radiation through the air. For example, depending on the temperatureof the air in the operating room, the period of time that it takes fromthe instant that the temperature compensation emitter 304 is actuated toradiate a signal until the instant that each of the microphones of thearray 300 receives the emitted signal will vary. The SAR program 316knows, through calibration, the distance between the temperaturecompensation emitter 304 and each of the microphones of the array 300.Therefore, the SAR program 316 can immediately calculate the velocity ofthe signals being transmitted. This velocity establishes a reference fordetermining the position of the surgical probe 302 and the base ring306.

Next, the emitters of the base ring 306 are activated so that theposition of the base ring 306 can be determined. At this point, theemitters of the base ring 306 are successively energized and theradiation transmitted by these emitters is detected by the microphonearray 300. The signal generated by the microphones from this radiationis digitized and evaluated by the SAR program 316 to determine theposition of each of the emitters of the base ring 306. Once thepositions of the base ring emitters have been determined by the SARprogram 316, standard geometrical computations are performed by the SARprogram to determine the plane defined by the base ring 306 with respectto the microphone array 300.

Digitizer 312 then signals multiplexer 310 to provide the signalgenerated by signal generator 308 to the surgical probe 302. At thispoint, the emitters of the surgical probe 302 are successively energizedand the radiation transmitted by these emitters is detected by themicrophone array 300. The signal generated by the microphones from thisradiation is digitized and evaluated by the SAR program 316 to determinethe position of each of the emitters of the surgical probe 302. Once thepositions of the probe emitters have been determined by the SAR program316, standard geometrical triangulation is performed by the SAR programto determine the location of the tip of the surgical probe with respectto the microphone array 300.

Therefore, by using the 3D digitizer system, the position of the basering 306 and the position of the surgical probe 302 relative to the basering 306 can be determined by the SAR program 316. As noted above, thebase ring 306 is mounted to the reference ring 120 (FIG. 1C) and isessentially coplanar therewith so that the base ring 306 defines thereference plane RP of the scanned image coordinate system illustrated inFIG. 1D.

Computer 314 includes translational software 318 which then translatesthe coordinates of surgical probe coordinate system illustrated in FIG.2D into the scanned image coordinate system illustrated in FIG. 1D. As aresult of this translation, computer 314 has now determined theparticular scanned image of the preoperative scan on which the tip ofthe surgical probe 302 would be located. The system includes a tapedrive 320, accessed through a local area network (LAN) 321, in whicheach of the images of the preoperative scan are stored. The translatedcoordinates generated by translational software 318 are provided to thestereotactic image display software 322, also resident within computer314, and identify the particular scanned image which is to be viewed bythe surgeon. The identified image is selected by the stereotacticimaging system 324 which recreates the image from the data stored intape drive 320 and displays it on a high resolution display 326.Stereotactic image display software 322 and stereotactic image system324 may be any off-the-shelf system such as manufactured by StereotacticImage Systems, Inc. of Salt Lake City, Utah.

Referring to 3B, a perspective illustration of the microphone array 300,temperature compensation emitter 304, surgical probe 302 and base ring306 are illustrated. Microphone array 300 includes a plurality ofmicrophones 350, the outputs of which are connected to 3D digitizer 312.Adjacent to the microphone array 300 is a temperature compensatingemitter 304 which selectively emits signals used by the SAR program incalibration to determine the velocity of the radiation. For example, inthe Scientific Accessories Corporation Model GP-8-3D, a sonic digitizeris used. In this case, the speed of sound being transmitted from thetemperature compensation emitter 304 to the microphones 350 iscalculated by the SAR program to determine the speed at which the soundis being transmitted through the air. Since this system is very accurateand the speed of sound varies fairly significantly with respect to thetemperature of the air, the temperature compensation emitter 304 allowsthe 3D digitizer system to compensate for changes in the air temperaturein the operating room. Surgical probe 302 comprises a bayonet surgicalforceps modified to carry at least two sound emitters thereon which areessentially coaxial on axis 362 with the tip of the forceps. Theemitters are in line and immediately below the surgeon's line of sightthrough the forceps so that the line of sight is not blocked. Ingeneral, the microphone array 350 is attached to the operating lightabove the patient's head so that it is in direct line of sight with theforceps as they are being used by the surgeon. The microphones 350listen to the sound emitted from the sequential energization of theemitters 360 on the forceps. The SAR software 316 measures the time oftransmission from each of the sound emitters 360 on the forceps to themicrophones 350. By comparing these times, the position of both emitters360 and, therefore, the tip of the forceps can be calculated by the SARprogram 316.

Base ring 306 is affixed to the reference ring 120 attached to thepatient's head and is essentially coplanar with the reference pointsRP1, RP2 and RP3. Base ring 306 includes a plurality of emitters 370thereon which are connected to multiplexer 310 and energized by signalgenerator 308. Each one of these emitters 370 is sequentially energizedso that the radiation emitter thereby is received by the microphones 350of array 300. The emitters 370 are preferably positioned 900 apart withthe center emitter being located at the anterior of the head. Thispermits base ring 306 to be mounted around the head so that all threeemitters are in line of sight with the array. The resulting signals aredigitized by digitizer 312 so that the SAR program 316 is able todetermine the plane in which the emitters 370 are located. This planeessentially defines the reference plane because it is coplanar with thereference points RP1, RP2 and RP3. By determining the position of thereference plane, translational software 318 is now able to take thecoordinate position of the probe 302 and translate it from the surgicalprobe coordinate system of FIG. 2D into the scanned image coordinatesystem as illustrated in FIG. 1D. As a result, the particular scannedimage which corresponds to the position of the probe can be identifiedand displayed for viewing by the surgeon.

The surgical probe 302 is generally a bayonet cauterizing device whichhas a bundle of wire 364 attached thereto. Therefore, the wires requiredto connect the emitters 360 to the multiplexer 310 are part of thebundle of wires 364 which connect the forceps to its electrical powersource and the surgeon is familiar with handling such forceps connectedto a wire bundle. Therefore, there is no inconvenience to the surgeon inusing such a probe and the surgeon is familiar with handling such aforceps connected to a wire bundle.

Base ring 206 is one apparatus for determining and positioning thereference points RP1, RP2 and RP3 with respect to the microphone array300. An advantage of the base ring 306 is that each time the patient'shead is moved the base ring 306 is energized to define the referenceplane. This allows the surgeon to move the patient's head duringsurgery. Alternatively, the reference points RP1, RP2 and RP3 can beestablished by using a reference mode of the 3D digitizer 312. Inparticular, the tip of probe 302 is positioned on each of the referencepoints RP1, RP2 and RP3 and actuated to emit a signal to the microphonearray 300 so that the position of the tip can be determined at each ofthese points. This is performed during a reference mode of operation ofthe 3D digitizer 312 so that the SAR program 316 calculates, at the endof the execution of this mode, the position of the reference points RP1,RP2 and RP3. This requires that the reference points have to bereestablished before the position of the surgical probe is determined toavoid changes in the reference plane due to movement of the head. On theother hand, one advantage of this approach is that the use of thereference ring 120 may be eliminated. In particular, it is possible thatthe reference pins 122 can be permanently affixed to the skull of thepatient. For example, these pins may be radiolucent surgical screwswhich are embedded in the patient's skull and which have radiopaquetips. These screws would be affixed to the patient's skull beforesurgery and before the preoperative scanning so the radiopaque tipswould provide a constant reference during scanning and throughout thestereotactic surgical procedure. During the actual surgery, the probewould be used to indicate the position of each of the radiopaque tipsbefore the probe position was determined. By eliminating the need forthe reference ring 120, other advantages are also achieved. For example,generally the preoperative scanning must be done under anestheticbecause the reference ring 120 interferes with intubation. Therefore,intubation must occur before the reference ring is affixed to the skull.By eliminating the need for the reference ring 120 and using surgicalscrews to identify the reference points RP1, RP2 and RP3, thepreoperative scanning can be performed without the need for intubationand the anesthesia accompanying it. In one alternative embodiment, it iscontemplated that the emitters 370 may each be separately mounted to ascrew or other fixed structure positioned at one of the referencepoints.

In summary, this process according to the invention is illustrated inFIG. 3C and identifies the location of the tip of the surgical probe 202for the surgeon. Initially, the reference plane is determined byenergizing the base ring 306 or by positioning the probe 302 at thereference points (as described herein). Next, the surgeon positions theprobe in the position desired within the head. The emitters of the probeare then energized so that the probe position is measured and determinedin the surgical probe coordinate system (X₂, Y₂, Z₂). Next, thetranslational software 318 converts the surgical probe coordinate systeminto the scanned image coordinate system (X₀, Y₀, Z₀) so that the imagecorresponding to the probe position can be displayed.

Referring to FIG. 3D, a perspective illustration of a patient's head 390in a cradle 392 during the scanning process is shown. As will bedescribed below, optical scanner 380, having emitters 381 thereon, isemployed to determine the position of the head 390 relative to a cradle392 positioned on the head.

Referring to 3E, a perspective illustration of the microphone array 300,temperature compensation emitter 304, surgical probe 302 and opticalscanner 380 are illustrated. Microphone array 300 includes a pluralityof microphones 350, the outputs of which are connected to 3D digitizer312. The microphone array 300 provides a fixed frame of reference towhich the position of probe 302 is measured and to which the position ofthe head 390, relative to the cradle 392, is measured. As a result, theposition of the probe 302 relative to the head 390 at any instant intime can be determined.

Adjacent to the microphone array 300 is a temperature compensatingemitter 304 which selectively emits signals used by the SAR program incalibration to determine the velocity of the radiation. For example, inthe Scientific Accessories Corporation Model GP-8-3D, a sonic digitizeris used. In this case, the speed of sound being transmitted from thetemperature compensation emitter 304 to the microphones 350 iscalculated by the SAR program to determine the speed at which the soundis being transmitted through the air. Since this system is very accurateand the speed of sound varies fairly significantly with respect to thetemperature of the air, the temperature compensation emitter 304 allowsthe 3D digitizer system to compensate for changes in the air temperaturein the operating room.

Surgical probe 302 comprises a bayonet surgical forceps modified tocarry at least two sound emitters 360 thereon which are essentiallycoaxial on axis 362 with the tip of the forceps. The emitters are inline and immediately below the surgeon's line of sight through theforceps so that the line of sight is not blocked. In general, themicrophone array 300 is attached to the operating room light above thepatient's head so that it is in direct line of sight with the forceps asthey are being used by the surgeon. The microphones 350 listen to thesound emitted from the sequential energization of the emitters 360 onthe forceps. The SAR software 316 measures the time of transmission fromeach of the sound emitters 360 on the forceps to the microphones 350. Bycomparing these times, the position of both emitters 360 and, therefore,the tip of the forceps can be calculated by the SAR program 316.

Optical scanner 380 is generally located over the patient's head 390 andis used during scanning to establish the position of the head 390relative to the cradle 392 thereby to relate the frame of reference ofthe cross sectional scans to the forehead 394. Scanner 380 is also usedduring surgery to establish the position of the head 390 relative to thecradle 392 thereby to relate the frame of reference of the probe 302 tothe forehead 394.

During the preoperative scanning process as shown in FIG. 3D, when thecross sectional images of the head are created, the patient's head liestemporarily in cradle 392. The cradle includes an arc 393 of radiopaquematerial so that it appears in at least some of the cross sectionalscans. As a result, the arc 393 defines a plane relative to the head390. During scanning, this plane can be defined as the 0,0,0 plane forconvenience. After the head is placed in the cradle, optical scanner 380is used to establish the position of the cradle 392 and its attached arc393 relative to the forehead 394. In particular, the optical scanner 380scans both the forehead and the arc 393 of the cradle 392 and, viacomputer 396 employing forehead fitting software 398, determines theposition of the arc 393 of the cradle 392 relative to the forehead 394.The forehead fitting software may be any off-the-shelf or customsoftware which graphs a set of points so that a curve defining thecontour of the forehead can be calculated, a curve defining the arc canbe calculated, and a curve defining the relative position of theforehead and the arc can be calculated. Since the position of the crosssectional scans relative to the radioopaque arc 393 is known (becausethe cradle arc defines the 0,0,0 plane) and since the position of thearc 393 of the cradle 392 relative to the forehead 394 is known (becauseof the scanning by the optical scanner), then the position of the crosssectional scans relative to the forehead is known and can be calculatedby translational software 316.

During surgery, a base ring 306 is firmly affixed to the head. The basering 306 does not have to be positioned in the same location relative tothe head as the arc was during the scanning process when the crosssectional images were created. The base ring 306 used during surgeryincludes emitters 370 which communicate with the array 300 to establishthe position of the base ring 306. As a result, the base ring 306defines a plane relative to the head 390. After affixing the base ringto the head, optical scanner 380 is used prior to or during the surgeryto establish the position of the base ring 306 relative to the forehead394. In particular, the optical scanner 380 scans both the forehead andthe base ring 306 and, via computer 396 employing forehead fittingsoftware 398, determines the position of the base ring 306 relative tothe forehead 394.

Since the position of the probe relative to the base ring is known(because of communication via the array) and since the position of thebase ring relative to the forehead is known (because of the scanning bythe optical scanner), then the position of the probe relative to theforehead is known and can be calculated by translational software 318.Since the position of the cross sectional images relative to theforehead is also known (from the preoperative scanning process), the endresult is that the position of the probe relative to the cross sectionalimages is known so that the position of the tip of the probe on theclosest cross sectional image can be displayed.

Optical scanner 380 and computer 396 are a standard, off the shelfscanner used to scan an object to determine its three-dimensional shape.For example, a limb scanner such as PIXSYS Optical Scanner used todevelop three-dimensional models for artificial limbs may be used. Thescanner 380 emits a laser beam or other optical beam toward the arc 393and the forehead 394 and receives the light reflected there through anarray of linear chip cameras such as CCD (charge coupled device)cameras. By evaluating the position of the reflected light using thecamera array, the optical scanner 380, including a computer 396,determines the shape and, thus, the contour of the forehead 394, theshape of the arc 393 of cradle 392 and the relative position of theforehead and the arc 393. Computer 396 indicates to the translationalsoftware 316 of computer 314, which is a part of the system asillustrated in FIG. 3A, the position of the probe 302 relative to theforehead 394. The translational software 318 then coverts this indicatedposition into the coordinate system of the cross sectional scannedimages. As a result, the particular scanned image which corresponds tothe position of the probe can be identified and displayed on display 326(FIG. 3A) for viewing by the surgeon.

The surgical probe 302 is generally a bayonet cauterizing device whichhas a bundle of wire 364 attached thereto. Therefore, the wires requiredto connect the emitters 360 to the multiplexer 310 are part of thebundle of wires 364 which connect the forceps to its electrical powersource. Surgeons are generally familiar with handling such forcepsconnected to a wire bundle. Therefore, there is no inconvenience to thesurgeon in using such a probe and the surgeon is experienced withhandling such a forceps connected to a wire bundle.

One advantage of the optical scanner 380 is that it eliminates the needfor a ring or pins to be attached to the patient's head during thepreoperative scanning process. Each time the patient's head is placed ina cradle, the optical scanner 380 can be used to scan the head andcradle to redefine their relative position without the need for anycontact. The reference ring (i.e., arc) on the head is, therefore,temporary. By eliminating the need for a permanent reference ring 120 orreference pins RP1-RP3, other advantages are also achieved. For example,generally the preoperative scanning must be done under anestheticbecause the reference ring 120 interferes with intubation or it must bedone after pins are affixed to the head. Therefore, intubation mustoccur before the reference ring is affixed to the skull. By eliminatingthe need for the permanent reference ring 120 and/or reference pins, andby using the contour of the forehead to define a reference point, thepreoperative scanning can be performed without the need for intubationand the anesthesia accompanying it.

In summary, during the preoperative scanning process the patient simplylies in a U-shaped cradle attached to the end of a CT or MRI table.Above the patient's face is an arc providing the reference plane. Allscans are obtained with reference to and preferably parallel to this arcdefining the reference or base plane. The optical scanner relates theforehead contour to this arc so that the relation of the forehead to thescans is known.

In the operating room, the patient's head is again scanned with theoptical scanner but this time the arc over the patient's head is basering 306. The reference emitters attached to the base ring define theoperative reference system. Therefore, the forehead is again related tothe base ring by the optical scanner to define a new reference system;this time the new reference system is the operating room. The computerthen matches the forehead contours obtained in the operating room andthe scanning room to relate the two reference systems. In effect, theforehead is a “bridge” between the reference system of the preoperativescanner and the reference system of the operating room.

The cradle does not have to appear in the actual scans. The primarypurpose of the cradle is to keep the patient's head from moving so thatall scans are obtained with the same relationship to the arc.

Referring to FIG. 4, a flow chart of the operation of the translationalsoftware 318 is illustrated. Initially, the surgeon locates the probe302 in the position which is to be determined. (If a base ring 306 isnot being used to identify the location of the reference plane, theinitial step is for the surgeon to use the reference mode of the 3Ddigitizer 312 to identify the reference plane by locating the surgicalprobe tip at several points in the plane.)

The system initializes at step 400 so that translational software opensa window menu at step 402 of a multitasking program such as DESQ VIEWdistributed by Quarterdeck Office Systems of Santa Monica, Calif. Suchsoftware permits simultaneous execution of multiple software programs.In general, once a program is selected for actuation, it continues torun either in the foreground or in the background until deactuated.

The translational software continues initializing by selecting thestereotactic imaging system and actuating the stereotactic imagingsystem in the foreground by opening the stereotactic window at step 404.Thereafter, the translational software returns to the window menu atstep 406 moving the stereotactic image display software to thebackground and selects the digitizer window at step 408 to actuate thedigitizer in the foreground. The computer is then ready to be actuatedby the foot switch.

The surgeon then actuates a foot pedal or other switch which indicatesthat the system should perform a computation. Actuation of the footswitch is essentially the beginning of the start step 410. Uponactuation, the digitizer energizes calibration by the temperaturecompensation emitter 304 to determine the velocity of the sound waves,energizes the emitters of the base ring 306 to locate the referenceplane and energizes the emitters of the surgical probe 302 to locate theposition of the tip of the probe 302. The signals generated by themicrophone array are digitized so that the SAR program 316 determinesthe coordinates of the tip of the surgical probe. At step 412, thetranslational software 318 selects the coordinates from the SAR program.

Next, the window menu is again accessed at step 414 and the window menuswitches to the stereotactic image system software to the foreground atstep 416 to specifically control the operation of the stereotacticimaging system 324. At this point, the translational software 318 issuesan FI command to the stereotactic image display software 322 which inturn prepares the stereotactic imaging system 324 to accept coordinates.At step 420, the window menu is again selected so that at step 422 thecomputer switches the digitizer window into the foreground. At step 424,the digitizer window menu is accessed and coordinate translation isselected. At step 426, the digitizer begins calculating the coordinatesand at step 428 the coordinate calculation is ended. The translationalsoftware then returns to the digitizer window menu at step 430, switcheswindows to place the stereotactic image system software in theforeground at 432 to prepare it for receiving the coordinates and againreturns to the main window menu at step 434. Finally, the coordinateinformation is translated, including any necessary manipulation, andtransferred to the stereotactic image display software 322 at step 436which actuates the stereotactic imaging system 324 to select theparticular image from the tape drive 320 and display it on highresolution display 326. The stereotactic image display software 322instructs the stereotactic imaging system 324 to display the imageclosest to transferred coordinates and to display a cursor on thedisplay 326 at the coordinates which corresponds to the position of thetip of the probe. Thereafter, the computer 314 is in a standby modeuntil the foot switch of the surgeon is again actuated to execute thetranslational software beginning with the start step 410.

The translation that occurs in step 436 depends on the position of thesurgical probe coordinate system relative to the scanned imagecoordinate system and the units of measure. In the preferred embodiment,the systems are coaxial and the units of measure are the same so thatalgebraic adjustment is unnecessary. However, it is contemplated thatthe coordinates systems may not be coaxial, in which case translationwould require arithmetic and/or trigonometric calculations. Also, thesequence, e.g., (X₂, Y₂, Z₂), in which the coorinates are generated bythe digitizer may be different than the sequence, e.g., (X₀, Y₀, Z₀), inwhich stereotactic image system software receives coordinates.Therefore, the sequence in which the coordinates are transferred mayhave to be reordered.

Referring to FIG. 5A, a system employing an ultrasound localizer isillustrated. Reference character 500 refers to an ultrasound probe whichmay be used in the operating room to scan the brain. The ultrasoundprobe 500 includes a plurality of at least three emitters 502 whichcommunicate with the array 300 to define the plane in which theultrasound probe is scanning. Emitters 502 are energized via line 504 bymultiplexer 310 as in the other systems illustrated above. The radiationemitted by emitters 502 is received by array 300 to determine the planein which the ultrasound probe 500 is positioned. The ultrasound probe isalso connected via line 506 to a computer which analyzes the ultrasoundscanning and provides the analyzed information to a work station 510which displays the scanned image. Since the array 300 can determine theposition of the ultrasound probe 500 at any point in time, via digitizer312, the particular plane of the image displayed on work station 510 isknown. The position of the head of the patient can be determined byattaching a base ring with emitters to the head, as noted above, or byscanning the forehead with an optical scanner having emitteres thereon,as noted below.

For example, such an ultrasound image is illustrated in FIG. 5C. Thesurgeon can then call up the similar image on the display 326 of thestereotactic imaging system 324 such as illustrated in FIG. 5B.Alternatively, computer 508 may be linked to the stereotactic imagingsystem 324 directly to define the particular image plane illustrated onwork station 510 so that display 326 can display the correspondingscanned image. As a result, the image from the ultrasound system, asillustrated on work station 510, is shown on one monitor and may becompared to a cross section to the images obtained either by CT, MRI orPET scanning. The cross section through the three dimensional data setas developed by the ultrasound system is determined by a high speedgraphics work station, such as manufactured by Silicon Graphics. Thisallows the interpretation of the ultrasound scans as the anatomy fromthe MRI, CT or PET scans can be seen directly. Furthermore, theultrasound system allows scanning in the operating room. Since the braintissue is elastic and the position of various tissue may change fromtime to time, use of an ultrasound scan in the operating room permits amore definite localization of various brain tissues.

Alternatively, the system may be used for determining a position of theultrasound probe relative to a head of a body of a patient. The probe500 is positioned to scan the head 394 with an array 300 positionedadjacent the probe. At least three emitters 502 permit determination ofthe position of the ultrasound probe relative to the array. Opticalscanner 380, having emitters 381 (FIG. 3D) permit determination of theposition of the head relative to the array. Computer 396 translates theposition of the ultrasound probe into a coordinate system correspondingto the position of the head.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:
 1. A system for determining the position of a bodypart, the system comprising: a base fixed in relation to a body part; alight source positionable to direct light at the base and a surface ofthe body part; an array of sensors located remotely from the body partand positioned to receive light reflected from the base and the surfaceof the body part; a processor in communication with the array, theprocessor is configured to determine the position of the body partrelative to the array based on the light reflected to the array; anadditional array of sensors; and base reference points fixed relative tothe base, the base reference points in communication with the additionalarray of sensors, wherein the processor is in communication with theadditional array of sensors and the position of base is known based onthe communication between the base reference points and the additionalarray of sensors.
 2. The system of claim 1, further comprising a memorystoring previously taken scan images of the body part, the surface ofthe body part correlated to the scan images, wherein the processor isconfigured to correlate the surface of the body part to the location ofthe surface of the body part in the scan images.
 3. The system accordingto claim 1, further comprising a probe and probe reference points fixedin relation to the probe, the probe reference points in communicationwith the additional array of sensors, wherein the processor isconfigured to calculate the position of the probe relative to the base,thereby calculating the position of the probe relative to the body part.4. The system according to claim 3, further comprising a memory storingpreviously taken scan images of the body part, the scan imagescorrelated to the surface of the body part, and wherein the processor isconfigured to correlate the location of the body part to the location ofthe body part in the scan images.
 5. The system according to claim 1,wherein the light source comprises an optical scanner.
 6. The systemaccording to claim 1, wherein the array of sensors comprise linear chipcameras.
 7. A system for determining the position of a body part insurgical space, the system comprising: a base fixed in relation to abody part, the position of the base in surgical space being known; alight source being positionable to direct light at the base and asurface of the body part; an array of sensors positioned to receivelight reflected from the base and the surface of the body part; aprocessor in communication with the array, wherein the processor isconfigured to calculate the position of the body part relative to thebase based on the light reflected to the array, thereby calculating theposition of the body part in surgical space; an additional array ofsensors; and base reference points fixed in relation to the base, thebase reference points in communication with the additional array,wherein the processor is in communication with the additional array ofsensors and the position of the base in surgical space is known based onthe communication between the base reference points and the additionalarray of sensors.
 8. The system according to claim 7, further comprisinga memory storing scan images of the body part in a scan space, whereinthe processor is configured to correlate the position of the body partin surgical space to the position of the body part in scan space.
 9. Thesystem according to claim 8, wherein the base is fixed in a knownposition in surgical space.
 10. The system according to claim 8, furthercomprising a probe and probe reference points fixed in relation to theprobe, the probe reference points in communication with the additionalarray of sensors, wherein the processor is configured to calculate theposition of the probe in surgical space, thereby determining theposition of the probe relative to the body part, and wherein theprocessor is configured to correlate the position of the probe relativeto the body part in surgical space to the position of the probe relativeto the body part in scan space.
 11. A system for correlating theposition of a subject located in a first coordinate system to theposition of the subject in a second coordinate system, the systemcomprising: a memory storing scan images of the subject located in aknown position in the second coordinate system; a base located in aknown position in the first coordinate system, the base fixed relativeto the subject; a light source positionable to direct light at the baseand a body part of the subject; a light sensor positionable to receivethe light reflected from the base and the body part; a processor incommunication with the light sensor; an additional array of sensors; andbase reference points fixed in relation to the base, the base referencepoints in communication with the additional array, wherein the processoris in communication with the additional array of sensors and theposition of the base in the first coordinate system is known based onthe communication between the base reference points and the additionalarray of sensors; the processor configured to determine the position ofthe body part relative to the base based on the light received by thelight sensor thereby determining the position of the body part in thefirst coordinate system, and the processor configured to correlate theposition of the body part in first coordinate system to the position ofthe body part in the second coordinate system.
 12. The system fordetermining a position relative to a body of a patient, the systemcomprising: body reference points fixed in relation to the body, thebody reference points radiating signals indicating the position of thebody reference points; a memory having stored images of the body, theimages including reference images correlatable to the body referencepoints; a probe; probe reference points fixed in relation to the probe,the probe reference points radiating signals indicating the position ofthe probe reference points; a digitizer in communication with the bodyand probe reference points to determine the position of the bodyreference points and the probe reference points based on the signalsradiated by the body reference points and the probe reference points; acomputer in communication with the digitizer and the memory, wherein thecomputer is configured to determine the position of the body in theimages of the body, to determine the position of the body relative tothe body reference points, to determine the position of the proberelative to the probe reference points, to determine the position of theprobe relative to the body, and to translate the position of the proberelative to the body to the position of the probe relative to the bodyin the images of the body; and a display of the images of the translatedposition of the probe relative to the body.
 13. The system of claim 12wherein the computer is configured to translate the position in responseto the position determinations.
 14. A system for determining a positionrelative to a body of a patient, said system comprising: a referencefixed in relation to the body, the reference being energizable toproduce signals; a medical instrument located remote and apart from thebody and positionable relative to the body, the instrument beingenergizable to produce signals; a receiver in communication with thereference and instrument, the receiver configured to receive the signalsproduced by the reference and the instrument; and a processor incommunication with the receiver, the processor configured to determinethe position of the body relative to the reference, to determine theposition of the instrument relative to the reference based on thesignals received by the receiver, and to determine the position of theinstrument relative to the body based on the position of the bodyrelative to the reference and the instrument relative to the reference.15. The system of claim 14, further including a memory having storedimages of the body, the images including reference images correlatableto the reference and wherein the processor is configured to thedetermine the position of the body in the images of the body andtranslate the position of the instrument relative to the body to theposition of the instrument relative to the body in the images of thebody.
 16. The system of claim 14, further including a display fordisplaying the position of the instrument relative to the body.
 17. Thesystem of claim 14 wherein the processor is configured to translate theposition in response to the position determinations.
 18. A method forindicating a location relative to the body of a patient, the methodcomprising: positioning an instrument relative to a body of a patient,the instrument in communication with an array positioned remote andapart from the body and the body having fixed reference points incommunication with the array; communicating signals from the instrumentwith the array; communicating signals from the reference points to thearray; receiving at the array the signals from the instrument and thereference points; processing the signals received at the array with aprocessor in communication with the array to determine the position ofthe instrument relative to the reference points and the body; retrievingwith the processor scan images of the body from a memory; determiningwith the processor the position of the body in the images of the body;correlating with the processor the position of the instrument relativeto the reference points and the body to the images of the body; anddisplaying on a display the image of the instrument relative to the bodyin the images of the body.